The present invention relates to a magnetic filter adapted to be mounted in a gear box or the like of various types of vehicles.
A magnetic filter of the above-mentioned type in the prior art is shown in FIG. 4. In this figure, at a tip end of a screw rod portion (c) projected from a fastening head portion (b) in a screw plug (a) is formed a cylindrical recess (d) directed in the axial direction of the screw plug (a), a cylindrical permanent magnet piece (e) having magnetic poles at its opposite end surfaces (the top and bottom surfaces as viewed in FIG. 4) is fixedly fitted in the cylindrical recess (d), and a ring (f) made of non-magnetic material such as aluminium is interposed between the cylindrical permanent magnet piece (e) and an inner circumferential surface of the cylindrical recess (d).
In the case of the above-described magnetic filter in the prior art, since the size of the permanent magnet piece is limited by the size of the screw plug, it has been impossible to select a large surface area of the permanent magnet piece, and hence an adsorbing power for iron powder has been limited.
Consequently, as shown in FIG. 7 an improved magnetic filter has been proposed in which a permanent magnet piece (e) is constructed of an elongated rod-like piece, the tip end of the permanent magnet piece (e) being projected in the axial direction from the screw rod portion (c) to increase the surface area of the permanent magnet piece (e) so that an adsorbed amount of iron powder may be increased.
However, in the case of the magnetic filter shown in FIG. 7, due to the fact that the distance between the magnetic poles at the top and bottom ends of the above-described rod-shaped permanent magnet piece (e) is long and hence a length of the magnetic flux is large, the retaining force for iron powder is reduced, and even with a small impact, iron powder adsorbed to the magnetic pole would drop out. Moreover, on the outer circumference of the cylindrical permanent magnet piece (e) is formed a pole gap (H) where a magnetic flux is not generated, over a wide region, and iron powder cannot be adsorbed to such a pole gap (H).
Furthermore, since the magnetic flux extends in the axial direction of the permanent magnet piece (e), iron powder would be adsorbed also on a head portion (b) and a screw rod portion (c) of the screw plug (a), resulting in faulty effects.
Still further, according to this proposed design, although it is contemplated to increase an adsorbed amount of iron powder by elongating the permanent magnet piece (e) and increasing its surface area, practically, due to the above-mentioned large pole-gap, such an effect is small.
In addition, if a gear (Z) or the like exists in the proximity of the permanent magnet piece (e), there occurs a disadvantage that the gear (Z) is magnetized by the magnetic flux emanating from the magnetic pole of the permanent magnet piece (e) and hence iron powder would adhere to the gear (Z).